The present invention relates to a digital automatic gain control (AGC) circuit technology used for an address information demodulation circuit for demodulating address information recorded on a DVD (digital versatile disk) by phase modulation of a wobble signal.
On recordable DVD+Rs, rewritable DVD+RWs and the like among DVDs of various standards, address information is recorded in advance by an address in pre-groove (ADIP) method in which address information is embedded in a wobble formed by meandering a groove in a sine-wave shape. By detecting the wobble signal, high-precision address detection is attained.
The wobble signal from a DVD is binary phase-modulated by binary phase shift keying (BPSK). In an address information demodulation circuit, a technology of correcting a phase change (delay) caused by a band pass filter (BPF) of a carrier generation circuit is known (Japanese Laid-Open Patent Publication No. 2001-126413).
A technology of digitizing an AGC circuit for demodulation of BPSK is also known. In this technology, an input BPSK signal is converted to digital signals by sampling the BPSK signal at positions different in phase by a quarter period from each other. Peak-to-peak values of the signals are compared with each other, and a gain for the input BPSK signal is computed from a larger peak-to-peak value, and fed back to thereby control the peak-to-peak value of the input BPSK signal at a predetermined value (see Japanese Laid-Open Patent Publication No. 8-335957).
The wobble signal received from a DVD varies in amplitude in many cases. Therefore, conventionally, a problem arises that such amplitude variation still remains even after gain adjustment with a large time constant is performed in an analog circuit.